In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal, wherein a charge coupled device (CCD) is an element in which individual MOS (Metal-Oxide-Silicon) capacitors are very close to each other and a charge carrier is stored in the capacitor and is transported, and a CMOS image sensor is an element employing a switching scheme in which MOS transistors corresponding to the number of pixels are manufactured using a CMOS technology using a control circuit and a signal processing circuit as a peripheral circuit and output is sequentially detected using the MOS transistors.
In such an image sensor, color filters are arranged above a photodetection unit that receives light from an exterior and generates and accumulates photocharge, and a color filter array (CFA) may include three colors of a red, a green, and a blue, or three colors of a yellow, a magenta, and a cyan.
Basically, in order to express a color, a CMOS image sensor generally includes R (Red), G (Green), and B (Blue) pixels. However, the image sensor has a characteristic responding to light of an infrared area not visible by the human's eyes. Therefore, it is necessary to block light of a visible ray area and to allow only light of an infrared area to pass through, and in such a case, an infrared pixel is additionally used.
As described above, in order to optimize light efficiency of a CMOS image sensor including an infrared pixel, it is necessary to optimize microlens and filter layers for each pixel.
FIG. 1 is a diagram illustrating the arrangement of a unit pixel of a conventional CMOS image sensor including an infrared pixel, and FIG. 2 is a diagram schematically illustrating the section of the unit pixel of the conventional CMOS image sensor including the infrared pixel.
As illustrated in FIG. 2, the unit pixel of the conventional CMOS image sensor has a structure in which a photodetection layer 120, a metal interconnection layer 130, an insulating layer 140, a color filter layer 150, a transparent planar layer 160, and a microlens layer 170 are sequentially stacked on a semiconductor substrate 110.
The unit pixel of the conventional CMOS image sensor including the infrared pixel includes four red, green, blue, and infrared (IR) pixels as illustrated in FIG. 1, and color filters and an IR filter corresponding to the respective pixels are generally formed with no vertical stepped portion as illustrated in FIG. 2.
In general, according to the characteristics of a color filter, a difference occurs in crosstalk characteristics or purity characteristics according to the stacked thickness of the filter as illustrated in FIG. 4. For example, it can be understood that when the stacked thickness of the filter increases, crosstalk in a wavelength band to be cut off is reduced.
In the image sensor including the red, green, blue, and infrared (IR) pixels, each pixel shows spectral characteristics as illustrated in FIG. 3. In the IR pixel, a signal generated in a visible ray wavelength band, particularly, a band of 400 mm to 650 mm, causes crosstalk to degrade a depth of field (hereinafter, referred to as ‘DOF’) function in the infrared (IR) pixel. Also in the R, G, B pixels, the same phenomenon occurs.
In order to suppress crosstalk characteristics in a undesired wavelength band, it is necessary to improve filtering characteristics of a color filter. However, there is a limitation in arbitrarily changing the filtering characteristics so long as unique characteristics of color filter material do not change.
Furthermore, as illustrated in FIG. 2, when the color filter is formed with no vertical stepped portion, there is a limitation in thickness adjustment according to characteristics such as viscosity of material, resulting in the occurrence of purity reduction due to crosstalk of another color in a corresponding pixel.